1. Field of the Invention
The present invention relates to an apparatus for controlling the distribution of drive power for motor vehicles, and more particularly to an apparatus for controlling the distribution of drive power for four-wheel-drive motor vehicles whose four wheels, i.e., main and auxiliary drive wheels, are drivable by the engine.
2. Description of the Relevant Art
Some recent motor vehicles such as four-wheel-drive motor vehicles whose front and rear wheels are drivable by the drive power from the engine incorporate an apparatus for controlling the ratio of distribution of the drive power to the front and rear wheels depending on the running conditions of the motor vehicle, so that the motor vehicle exhibits improved running performance.
One such motor vehicle with drivable front and rear wheels is known from Japanese Laid-Open Patent Publication No. 62(1987)-203826, for example. The disclosed motor vehicle has a transmission disposed between the front and rear wheels and operable by a hydraulic actuator for speed changing operation. The hydraulic pressure supplied to the hydraulic actuator is controlled by a control valve to vary the ratio at which the drive power from the engine is distributed between the front and rear wheels.
The speed reduction ratio, i.e., the drive power distribution ratio, between the front and rear wheels is controlled by the hydraulic actuator. Therefore, if the control valve for controlling the hydraulic pressure supplied to the hydraulic actuator or a hydraulic system between the control valve and a pump for supplying the hydraulic pressure to the actuator fails for some reason, then it is impossible to control the drive power distribution ratio.